The attainment of high-temperature superconductivity with a new class of superconducting materials is of immense scientific and technological importance. Many members of this new class of superconducting materials belong to the family of ceramics called "perovskites." Typically, perovskites are described by the general formula ABX.sub.3 and consist of cubes made up of three distinct elements which are present in a 1:1:3 ratio. The perovskite structure is similar to the naturally occurring calcium titanate structure, CaTiO.sub.3, characterized by at least one cation much larger than the other cation or cations.
In late 1986, the superconducting properties of certain ceramic defect oxide type materials, which materials are variations of the typical perovskite class of inorganic structures, were observed by Bednorz and Mueller. The Bednorz and Mueller work was based upon materials developed by Michel and Raveau. The materials which Bednorz and Mueller observed contained lanthanum, barium, copper, and oxygen, and were reported to be superconducting at a temperature of about 30 degrees Kelvin. Continued work in the field resulted in the increase of the critical temperature, T.sub.c (the temperature at which electrons or holes are able to move through a material without encountering any resistance to that motion), by the substitution of yttrium for lanthanum. Upon analysis, the superconducting composition was found to be a perovskite ceramic defect oxide of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 type, an orthorombically distorted perovskite. Further work with this phase effectively raised the critical temperature to a few degrees above 90 degrees Kelvin (a temperature above the atmospheric boiling point of liquid nitrogen).
Other perovskite phase materials have a recurring crystallographic unit cell structure including substantially parallel a and b planes spatially disposed along and substantially parallel to the c-axis thereof. These other perovskite phase materials include many compositional variations over the basic ABX.sub.3 formula. Such compounds as magnesium-iron silicate, calcium uranium oxide, Ca.sub.2 FeTiO.sub.y, and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y are further examples of perovskite phase materials.
The superconducting perovskite type materials are ceramic based defect oxides. That is, the superconducting phases of the perovskite type materials are solids in which different kinds of atoms occupy structurally equivalent sites, and where, in order to preserve electrical neutrally, some sites are unoccupied, or vacant. Since these vacancies may be filled with mobile oxygen atoms, only local order is prevalent with periodicity existing along the planes. These vacant sites form lattice defects, which defects have, generally, profound effects upon the electrical parameters of the superconducting material and more particularly upon the oxidation states of the copper atoms in the unit cells thereof.
Heretofore, single crystal superconducting perovskite type films could only be grown on a "template," i.e., an underlying substrate characterized by substantially the identical crystallographic lattice structure as that of the superconducting film. The superconducting film deposited on this "template" can thereby be epitaxially grown according to the lattice structure of the substrate. Materials, such as strontium titanate and lanthanum aluminate, which have lattice structures matched to the lattice structures of perovskites, are thus utilized as preferred substrates for the epitaxial growth of superconducting perovskite ceramic-oxide type films. However, because these perovskite substrates are very expensive and provide limited surface area upon which to deposit superconducting material, they have limited practical commercial importance.
Typically, non-epitaxially grown superconducting perovskite ceramic defect oxide-type films are polycrystalline in nature. That is to say that they are formed of individual superconducting grains columnarly arising from the underlying substrate. In prior work, efforts of the instant inventors at aligning these individual grains have resulted in spatial alignment only along the c-axis of the unit cells thereof (See commonly assigned U.S. patent application Ser. No. 442,380 filed on Nov. 28, 1989, entitled "Method of Aligning Grains of a Multi-Grained Superconducting Material). While such c-axis alignment provided increased current flow as compared to randomly oriented superconducting material, it failed to provide the high current carrying capacity originally anticipated. The type of columnar growth present in typical polycrystalline superconducting material characterized by such c-axis orientation produces grain boundaries between individual grains. Current flowing along the a-b plane cannot travel very far before encountering the high angle grain boundaries separating adjacent crystallites, which effectively restrict current flow thereacross.
One of the instant inventors also previously disclosed Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 superconducting films which were modified by the addition of a "parametric modifier" element to fill structural vacancies. (See commonly assigned U.S. patent application Ser. Nos. 043,279 filed Apr. 27, 1987, 444,487 filed Nov. 27, 1989 and 542,620 filed Jun. 22, 1990, entitled "Parametrically Modified Superconductor Material." The researchers at Energy Conversion Devices, Inc. realized that in order to achieve yet higher critical temperatures, it would be necessary to develop a superconducting material in which the chemistry thereof was engineered so as to alter the local chemical and electrical environment. For example, it has been established that the mobility of oxygen atoms in the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 ceramic based systems is very high and therefore the location of those mobile oxygen vacancies at any point in time contributes to the presence or absence of high T.sub.c superconducting phases. It is this oxygen mobility and changing local environment which results in the unstable electronic properties of this class of superconducting materials. Energy Conversion Devices, Inc., found that the addition of the very small and highly electronegative fluorine atoms effectively occupied lattice sites in the ceramic based fluoro-oxide class of superconducting materials so as to cause "grid lock" and provide an impediment to the mobility of oxygen atoms. The result was a stabilized high critical temperature superconducting material. Zero resistance evidence was provided of superconducting phases in parametrically "modified" materials as high as 155 to 168 Kelvin. The parametric modifier serves as a catalytic agent to promote grain alignment and to promote film growth along the a-b basal plane and inhibit c-axis growth.
The ability of high T.sub.c superconducting materials to carry high critical current densities is not only of great scientific importance but has immense economic significance. Initially, researchers were not sure of the current density carrying capabilities of the high critical temperature phases of these high T.sub.c superconducting materials. However, this doubt was resolved by scientists at various laboratories throughout the world who demonstrated experimentally that the high T.sub.c ceramic defect oxide superconducting materials could carry current densities exceeding 10.sup.8 amperes per square centimeter at 77K. This was determined by measuring the current density carried by either a single crystal or an epitaxially grown thin film of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 material in a direction parallel to the a-b plane, i.e., perpendicular to the c-axis of the unit cell thereof. However, the single crystal and the epitaxial thin films were found to be strongly anisotropic and could only carry about 10,000 amperes per square centimeter of current in a direction other than along said basal planes.
These experiments indicate that the high T.sub.c grains of the polycrystalline superconducting material are highly unaligned and the current density is limited by the high angle grain boundaries which result from columnar growth of the relatively small grains thereof. This is contrary to previous thinking to the effect that the alignment of the discrete grains of the polycrystalline superconducting material only along the c-axis, vis-a-vis, the basal plane, would be sufficient to produce materials having high current carrying capacities. It is now clear that alignment of the unit cells in the a-b direction as well as in the c-direction of the superconducting material is required in order to obtain an aligned current path and provide a superconducting material capable of carrying high current densities.
The extremely anisotropic nature of the high critical temperature superconducting materials, where the current flows preferentially along the Cu-O plane, and the strong chemical reactivity of the material have been the major stumbling blocks in the commercial development of high T.sub.c superconducting materials. It is clear that, randomly oriented polycrystalline film, tape or wire cannot be utilized to carry the high current densities necessary for most commercial applications. Up to now, the high current carrying capability of the high T.sub.c superconducting materials has only been demonstrated with tiny single crystals or on films epitaxially grown on perovskite substrates of the type characterized by a lattice mismatch of less than 2%, such as SrTiO.sub.3, LaAlO.sub.3, LaGaO.sub.3, etc. However, these substrates are too costly for use in the fabrication of commercial devices, are available only in small wafers, and/or possess high dielectric constants and high dielectric losses. Further, such free standing single crystal superconducting materials are many times too small, inherently brittle and inflexible to be of commercial significance.
Accordingly, flexible, "epitaxial-like" films grown on electronic quality, inexpensive substrates must be utilized in order to make it commercially feasible to fabricate wire or other flexible superconducting material. Further, in numerous commercial applications, it is necessary that the superconducting material be grown on top of a metallic, highly conductive substrate, such as copper, silver or gold, to avoid a catastrophic failure in the event the superconducting material reverts back to its normal state.
Therefore, an urgent and long felt need exists for a method of growing single crystal, "epitaxial-like" thin films of flexible, high T.sub.c, superconducting material on inexpensive non-perovskite substrates, such as glass, metals, synthetic polymers and the like. This need is particularly great if those superconducting films are to be capable of (1) providing high current carrying capacities and (2) being grown in an inexpensive, roll-to-roll, mass production process. It is to satisfy these crucial and long felt needs that the instant invention is directed.
It is to be understood that in superconducting materials of the type discussed herein a phenomenon termed "A-B twinning" frequently occurs. In materials of this type, the a and b crystalline axes are quite similar in physical properties; consequently they can substitute for one another in a crystal and this crystal is termed "twinned." Twinning does not adversely affect the properties of the superconducting material and as used herein, the term "single crystalline" includes twinned materials. It is also to be noted that superconducting materials typically exhibit superconductivity at low temperatures; thus a "superconducting material" can only exist, in a literal sense, at temperatures below the T.sub.c of that material. Nonetheless, the term "superconducting material" as used herein includes those materials which can manifest a superconducting effect even when they are above their Tc and hence not really superconducting.